1. Field of the Invention
This invention relates to electronic article surveillance and in particular it concerns novel responders and novel responder detection systems as well as novel methods for making and using same.
2. Description of the Prior Art
U.S. Pat. No. 4,623,877, in the name of Pierre F. Buckens and assigned to the assignee of the present invention, shows and describes an electronic article surveillance system in which articles of merchandise, e.g. books, clothing, etc., are protected from theft or other unauthorized removal from a protected area by securing to the articles a responder, otherwise known as a target, and providing a target monitor at each exit from the protected area. The target comprises an elongated strip of magnetically soft, i.e. easily saturable, low coercivity material. A transmitter and a receiver are provided with antennas located at the exit from a protected area. The transmitter generates a continuous alternating magnetic field at the exit; and when an article with a target attached is carried through the exit, the target is magnetically saturated successively in opposite directions by the alternating magnetic field and thereby produces distinctive disturbances of the field. The thus disturbed field is received by the receiver which in turn produces corresponding electric signals. The receiver then processes these electric signals and selects those corresponding to the particular distinctive disturbances produced by the targets. These selected signals are then used to actuate an alarm.
U.S. Pat. No. 5,029,291 in the names of Y. Peter Zhou et al, and also assigned to the assignee of the present invention, shows and describes a novel sensor element which is suitable for use as a responder or target in an electronic article surveillance system of the general type shown and described in the above mentioned patent to Buckens. The sensor element of the Zhou et al patent has a magnetic hysteresis characteristic having a different slope in one direction of magnetization than in the opposite direction of magnetization. Also, the slope in one direction of magnetization is very steep; and when the responder is subjected to a changing magnetic field, it produces disturbances of that field in the form of very sharp pulses.
The sensor element of the Zhou et al patent comprises a first layer of a cobalt-iron alloy containing a metalloid element such as boron and/or silicon and a second layer comprising a complex metal-metalloid compound formed from the first layer with the first and second layers being exchange coupled. As described in the patent, the sensor element is made by placing an element comprising the first layer as a substrate in a furnace containing an oxidizing atmosphere and heating the element at a temperature of 260.degree.-420.degree. for a period of two hours to eighty hours, until a film forms on the substrate. During the heating process electrical coils, such as Helmholtz coils, are energized to produce a magnetic field of about 0.3 oersteds along the length of the oxidized substrate while the substrate is isolated from all other magnetic fields, including the earth's magnetic field. This magnetic field is maintained until the furnace is cooled down.